Embodiments of the present invention relate generally to semiconductor devices, and more particularly to a structure and method of testing degradation of semiconductor devices.
Fabricated semiconductor devices with small physical dimensions may suffer from significant parameter variations. Additionally, as device dimensions become smaller these parameter variations may become larger. Reliability and parameter variation of these devices are vital to further expand applications of electronic circuitry. Measurement of a semiconductor device's degradation using a “stress” degradation test is utilized as a way to qualify fabricated devices for lifetime prediction. The degradation test may consist of applying a “stress” voltage to the device, followed by measuring the degradation resulting from application of the stress voltage. Application of the stress voltage may result in an increase of the device's threshold voltage, and in a reduction of carrier mobility within the device's channel, both of which lead to decreased device current. The reduction in device current is measured over a period of time for each device being tested to provide an indication of the device's reliability.
While conventional degradation tests may apply a static DC voltage to stress the device being tested, this may not adequately simulate what the device would actually experience in a digital circuit under real-use conditions. Under real-use conditions, device voltages may only remain on for very short time durations while the static DC stress may be applied for long time durations. Additionally, hot carrier degradation may also get exaggerated due to self-heating of the device with static DC voltage testing, especially when the applied DC stress pulses are longer than a thermal time constant, for example when current pulses are applied for a duration greater than hundreds of nanoseconds. Also, dynamic effects such as recovery between pulses may not be detectable when testing is done with static DC stress. As an alternative, stress may also be measured by applying short pulses. However, short pulses may introduce a measurement problem: in order to stress the device for a long enough time to measure degradation using short pulses, the measurement time may need to be impractically long.